Recently in automobile field, techniques of weight reduction have been developed: panels are more likely to be made of aluminum, and chassis and main bodies are improved to have high-tensile strength. Demand for weight reduction is now increasing in the current situation where international awareness of global warming caused by emission from vehicle becomes stronger, and reduction of CO2 contained in exhaust gas became a big issue.
In the automobile field, techniques for reducing CO2 have been studied mainly towards the following two directions: a development of new power sources, such as hybrid engine, fuel cell motor system and battery motor system; and an improvement of existing engines.
Major techniques for improving existing engines include “improvement of combustion method” and “energy-saving of power source by weight reduction”. Especially in Europe, a new fuel regulation will be introduced in 2010, and in order to comply with this regulation, automobile companies are engaged in developments of immediate weight reduction of whole car body.
These demands also include weight reduction of springs used in the car. For example, Patent Document 1 discloses a method for producing steel pipe having no seam (which corresponds to “seamless steel pipe” of the present invention) for springs in which a spring steel material is heated and subjected to Mannesmann-piercing, and then to mandrel mill rolling to thereby obtain a hollow raw pipe, which is then subjected to finish rolling.    Patent Document 1: Japanese unexamined patent application laid-open specification, No. H1-247532
However, in the method for producing seamless steel pipe in which Mannesmann-piercing is performed, there is a limitation on a minimum diameter of the rolling. For example, when a seamless steel pipe having a diameter of approximately 10 mm is produced from a hollow raw pipe, a multiple number of drawing steps and heating steps should be performed.
Especially when the material is a spring steel, which has a high hot deformation resistance, a section reduction per drawing should be considerably low as compared with general mild steel and the like, such as 20% (which specifically means that a pipe diameter of 100 mm2 becomes 80 mm2) Therefore, in the production method according to the above-mentioned patent document, there arises a problem that production efficiency becomes extremely poor.
Further, on a surface of the seamless steel pipe produced by the production method disclosed in the above-mentioned patent document, decarburization and rolling patterns may occur due to repeated hot processing, and there arises a necessity to peel (grind) an outer periphery surface and an inner periphery surface of the seamless steel pipe. Grinding may result in generation of flaws, especially in the inner periphery surface of the seamless steel pipe.
In addition, in the production method disclosed in the above-mentioned patent document, the seamless steel pipe is produced by Mannesmann-piercing, and when a material with high hardness is used, tools may frequently be damaged. Therefore, there was a problem that only a material with low hardness can be used, and thus only a seamless steel pipe with low endurance can be produced.
On the other hand, there is a method for producing seamless steel pipe by repeating heat treatment and press working. However, such a method is extremely poor in processing efficiency when producing a seamless steel pipe having a small diameter of, for example, approximately 10 mm, and thus feasibility is extremely low.
Though the above-mentioned patent document and the like proposed a production of seamless steel pipe, a seamless steel pipe having high hardness and a small diameter has not been produced in practice, due to the above-mentioned problems.
In a case of a hollow spring made of the conventional seamless steel pipe, there are disadvantages that a design stress should be made low and weight reduction effect is low, since a material strength of the seamless steel pipe itself is low and permanent transformation (permanent set) and fracture of the spring may occur under high stress.